Hydrogeochemical research and development at Mizunami Underground Research Laboratory up to 500m depth

This report summarizes the results of hydrogeochemical research and development in the Construction phase (Phase II) and the Operation phase (Phase III) of Mizunami Underground Research Laboratory Project up to 500m depth during 1st and 2nd Mid-term plan. We evaluated the 'Phase I - hydrogeochemical model' constructed in the Surface-based investigation phase (Phase I), and observed hydrogeochemical changes resulting from facility construction in the Phase II. Additionally we developed investigation techniques on redox condition, colloid/organics, etc. employed at gallery in the Phase III. Hydrogeochemical changes in sedimentary and granitic rocks resulting from facility construction have been caused by the change of mixing ratio among salinity-distinct groundwaters. Multivariate numerical analysis shows that the groundwater chemistry around drifts would change to that of shallow groundwater in the future. The know-hows were summarized to infer the hydrochemical changing processes during facility construction. Accumulation of knowledge regarding hydrogeochemical change and development of the simulation method of long-term hydrogeochemical condition in the facility closure and the post-closure phase is significant subjects for a future study. (author)